COMPUTATIONAL INVESTIGATION OF LIQUID SPRAY DISPERSION MODIFICATION BY CONICAL NOZZLE ATTACHMENTS

摘要

Liquid sprays characteristics such as the droplet size and the dispersion angle are determined by the atomizer design and the physical properties of the liquid and the surrounding gas. One of the options to change these characteristics is to attach a specially designed piece to the nozzle exit. While there can be a variety of shapes of such attachments, we chose a conical geometry to exploit its axial symmetry and, at the same time, obtain the results that can be generalized to other configurations. Thus, we investigate an addition of the conically shaped attachment to the premixed gas-assisted high-pressure atomizer with the previously developed numerical model. It is a two-fluid Eulerian-Eulerian model with a catastrophic phase inversion that was developed for compressible gas-liquid mixtures and can be applied to both the flow through the nozzle-atomizer and to the dispersion of the spray. The model also accounts for the break-up and coalescence effects of bubbles and droplets.Our investigation reveals that the conical nozzle attachments act as spray limiters by reducing the natural expansion angle of a spray. Also, the droplets produced by the nozzle with a conical addition tend to be larger than the ones obtained with a stand alone nozzle. The largest droplets were generated by the smallest attachment angle considered - 10°. With the increase of the angle, the spraying characteristics become closer to the ones of the stand alone nozzle. It can be concluded that the conical shape of the attachments with a relatively small angle may be used when higher jet penetrationand lower dispersion are desirable. The attachments with larger angles do not offer a substantial difference from the stand alone nozzle. Another important conclusion is that the dispersion of the jet is determined by the radial momentum transferred to the liquid before or immediately after the phase inversion takes place. Thus, for improved dispersion, the area where the atomization is taken place should not be restricted.